Metal surface cleaning



Patented Jan. 16, 1951 METAL SURFACE CLEANING James Harvey Noble, Rolfe Llewell Pottberg, and Urlyn Clifton Tainton, Baltimore, Md.; Rolfe Pottberg, administrator of said Urlyn C. Tainton, deceased, assignors, by direct and mesne assignments, to Freeport Sulphur Company, New York, N. Y., a corporation of Delaware No Drawing. Application June 2, 1944,

Serial No. 538,536

6 Claims. 1

This invention relates to an improved process for the cleaning, descaling and deoxidizing of metals, more particularly the ferrous alloys usually referred to as stainless steel. In present practice, these operations are usually carried out by immersing the metal in strong acids which operate by attacking the metal itself, and so bringing about a detachment of the surface scale or oxide.

The object of this invention is to provide a treating bath directly acting on the scale or oxide itself.

According to this invention the oxide coating on the metal is dissolved or converted into easily soluble form by immersion in a bath of a fused compound containing appropriate ingredients to bring about this conversion. For example, a ferrous alloy containing iron, nickel and chromium may be effectively treated by the use of a bath of fused sodium hydroxide containing an oxidizing agent such as sodium nitrate or sodium peroxide. This treatment removes either the whole of the scale or oxide, or any remaining is in such form as to be very readily soluble by a short treatment with a weak solution of acid.

Oxide films on hot rolled chromium nickel alloy steel may be removed by treatment in baths of fused sodium hydroxide containing oxidants in minor amount usually less than 10 for instance, as follows:

(a) Sodium nitrate, the sodium nitrate liberating oxygen according to the reaction 2NaNO3 NaNOz 02 the resulting films being easily removed in a subsequent short acid treatment.

(12) Sodium peroxide (Nazoz), plus a subsequent short acid treatment.

These baths in which further oxidation of some or all of the components of the oxide film occur, permit reaction between one or more of the metal oxides and the composition forming the fused bath, with complete or partial conversion, so that upon removal any residual film exists in a form which, possibly due to the removal of some of its original component oxides, and partly due to the changed form of the remaining compounds, is removed much more readily with a ds.

There is thus combined with a fused bath alkaline in character a further oxidizing effect favorable to the formation of higher oxides.

In certain cases it is advantageous to give material a preliminary treatment in an acid solution, 10% sulphuric or nitric acid being particularly effective, applied for a short period of two 2 to five minutes after which it is washed oil in hot water, drying before immersion in the fused compound.

The fused bath may consist of a compound or compounds of the desired basic properties, such as NaOH or NazC03, or may be composed largely of other suitable fusible substances which act as inert vehicles for a requisite percentage of such compound or compounds. Such reaction products as are not soluble in the fused bath itself, are removed by subsequent treatment after removal from the bath.

The reaction compounds usually formed, when they are not soluble in the bath itself, may show one or more of the following characteristics:

(1) They are decomposed by water, or are soluble in water.

(2) They are much more readily reacted upon by acids than the original oxide or oxides.

The oxidizing conditions of the fused bath can be produced in various ways. For example:

(I) By the addition of oxidizing substances such as nitrates, chlorates, peroxides, perchlorates. persulphates, etc., principally as salts of the alkali or alkaline earth metals;

(2) By formation by electrolytic means in the fused bath of oxidizing compounds, or of oxygen itself. For example, under controlled electrolytic conditions the formation of NazOz in fused baths of NaOH can be promoted.

The oxidant in the fused bath has proven very effective and the resulting compounds are either directly soluble in the bath or are more easily removable by after-treatment than the oxides in their original form.

The numerous advantages of the process as herein described are of great economic importance when considered in comparison with standard commercial present day practices of descaling, pickling and metal oxide film removal in general. Some of the more outstanding advantages are now briefly pointed out:

(1) The great reduction in time necessary for the cleaning operation by our process over the acid pickling process. The cycle of operations in our process can be completed in substantially under ten minutes. For example, in cleaning Ni-Cr alloy steel, an illustrative complete cycle for satisfactory oxide removal is as follows:

(a) 6-7 minutes fused NaOH+5% NaNO: 900 F.; or 4-5 minutes at 950 F.

(1)) Water quench, then (0) Treatment in 10% HCl at 180 for 1 minute followed by a 15 second treatment in 15% I-INO; at- 180, In the normal acid pickling process or combination acid processes, as applied to similar steels, the treatment time can run, for example, as high as 4 hours in 17 H2804 at 180 F.

(2) The normal acid pickling process leads to a great loss of metal by the acid attack on the base metal itself, this loss sometimes amounting to 60 lbs. of metal per short ton, whereas tests made on comparative metal loss caused by our process indicates actual metal sacrifice to fall substantially below 10 lbs. per short ton.

(3) The great problem of spent pickling acid disposal is practically eliminated by the use of our process, the actual acid requirements and consumption for the subsequent acid treatment in this process being very small.

(4) The fused bath utilized is very reactive with carbon, oxidizing it to removable products. Carbon is present in most annealed metal oxide films and is extremely difficult to remove by acid pickling. In these it is only thus completely removed by solution of the metal surrounding and beneath it.

(5) The short subsequent acid treatment required as an adjunct to the process diminishes the etching of the metal surface which is produced by the sustained action of acids over a long period as is normally required in acid pickling.

(6) The low cost of the chemicals required.

(7) The low cost of operation of the process.

(8) Simplicity of operation and of ingredients.

The metal alloys which are of general commercial application, and to which the invention can be beneficially applied, such as those of iron, chromium, nickel, manganese, cobalt, molybdenum, vanadium or tungsten, etc., in various combinations, are oxidized on the surface during certain stages of their fabrication into the customary forms required for industrial needs, such as billets, castings, sheet, rod, wire, etc. For example, heavy oxidation takes place during the annealing process, and this oxidized surface layer has in most cases to be removed before further fabrication or processing can be carried out.

The present process accomplishes this quickly and efliciently and is thus of wide scope and great practical utility.

We claim:

1. A process for the treatment of surface oxide films on ferrous metal article surfaces for the purpose of removing oxides therefrom comprising preliminarily subjecting the surface oxides to the action of an aqueous solution of a mineral acid, and then to a fused compound of the group consisting of sodium hydroxide and sodium carbonate and melting at a temperature of the order of 600 F. to 1560 F. and simultaneously with the latter subjecting said surface to the action of an oxidizing agent tending to develop further oxidation of said surface oxides while under the 4 action of said fused compound, and continuing said treatment to the modification of the oxides to a form easily removable in acids without depletion of the treated metal.

2. A process for the treatment of surface oxide films on ferrous metal article surfaces for the purpose of removing oxides therefrom comprising preliminarily subjecting the surface oxides to the action of an aqueous solution of a mineral acid, and then to a fused compound of the group consisting of sodium hydroxide and sodium carbonate and melting at a temperature of the order 600 F. to 1560 F. and simultaneously with the latter subjecting said surface to the action of an oxidizing agent tending to develop further oxidation of said surface oxides while under the action of said fused compound, continuing said treatment to the modification of the oxides to a form easily removable in acids without depletion of the treated metal, and thentreating said surfaces in an acid bath to remove remaining modified oxides.

3. A process for the treatment of surface oxide films on ferrous metal article surfaces as set forth in claim 1 in which the preliminary treating acid is of the group sulphuric and nitric.

4. A process for the treatment of surface oxide films on ferrous metal article surfaces as set forth in claim 1 in which the fused compound is caustic soda.

5. A process for the treatment of surface oxide films on ferrous metal article surfaces as set forth in claim 1 in which the oxidizing agent is an oxygen-producing salt.

6. A process for the treatment of surface oxide films on ferrous metal article surfaces as set forth in claim 1 in which the oxidizing agent is an oxygen-producing salt of the group consisting of the nitrate, peroxide and chlorate of sodium and of potassium.

JAlVIES HARVEY NOBLE. ROLFE ILEW'ELL POTTBERG. URLYN CLIFTON TAINTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,247,086 Crowe Nov. 20, 1917 1,859,734 George May 24, 1932 1,899,734 Stockton Feb. 28, 1933 2,311,099 Tainton Feb. 16, 1943 2,347,742 Keene May 2, 1944 2,395,694 Spence Feb. 26, 1946 FOREIGN PATENTS Number Country Date 332,608 Italy Dec. 3, 1935 466,661 Great Britain May 26, 1937 

1. A PROCESS FOR THE TREATMENT OF SURFACE OXIDE FILMS ON FERROUS METAL ARTICLE SURFCES FOR THE PURPOSE OF REMOVING OXIDES THEREFROM COMPRISING PRELIMINARILY SUBJECTING THE SRURFACE OXIDES TO THE ACTION OF AN AQUEOUS SOLUTION OF A MINERAL ACID, AND THEN TO A FUSED COMPOUND OF THE GROUP CONSISTING OF SODIUM HYDROXIDE AND SODIUM CARBONATE AND MELTING AT A TEMPERATURE OF THE ORDER OF 600* F. TO 1560* F. AND SIMULTANEOUSLY WITH OF THE LATTER SUBJECTING SAID SURFACE TO THE ACTION OF AN OXIDIZING AGENT TENDING TO DEVELOP FURTHER OXIDATION OF SAID SURFACE OXIDES WHILE UNDER THE ACTION OF SAID FUSED COMPOUND, AND CONTINUING SAID TREATMENT TO THE MODIFICATION OF THE OXIDES TO A FORM EASILY REMOVABLE IN ACIDS WITHOUT DEPLETION OF THE TREATED METAL. 